Exception when calling FeedsV20210630Api->getFeed: Client error: `POST https://api.amazon.com/auth/o2/token` resulted in a `401 Unauthorized` response:
{"error_description":"Client authentication failed","error":"invalid_client"}

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<b>Warning</b>:  Undefined variable $feed_document_id in <b>/home/amzgreatvaluesto/public_html/amz_sp/sp_api_get_feed.php</b> on line <b>30</b><br />
Exception when calling FeedsV20210630Api->getFeedDocument: Missing the required parameter $feed_document_id when calling getFeedDocument